1. Field of the Invention
The present invention relates to a light beam position detecting apparatus and, more particularly, to an apparatus for detecting a planar position of a light beam emitted on an object surface having an optically unidirectional structure such as a track of an optical disk.
2. Related Background Art
A push-pull method is known as a conventional method of controlling a position of an object surface irradiated with a light beam (this control method is referred to tracking hereinafter) in an information recording/reproduction apparatus using an optical recording medium such as an optical disk or an optical card.
Tracking by a push-pull method used in an optical disk apparatus will be described below.
FIG. 1 is a schematic view showing an optical system for explaining tracking by a push-pull method.
Referring to FIG. 1, an optical disk 1 has lands 2 and grooves 3, both of which are formed on an information recording/reproduction surface (to be referred to as an information surface hereinafter). The lands 2 and the grooves 3 constitute tracks. The optical system includes an objective lens 5, a condenser lens 6, and two-split sensor elements 8 and 9. A light beam emitted from a light source (not shown) is focused by the objective lens 5 on the information surface to form a beam spot 4. The light spot 4 is diffracted and reflected by the land 2 and the groove 3 which have unidirectivity in the x direction. The reflected beam is incident again on the objective lens 5 and is converted by the condenser lens 6 into detection light beams 7 which are then incident on the sensor elements 8 and 9. In this case, the two-split sensor elements 8 and 9 are located at a far-field position different from an image point 10 serving as a focal point.
FIGS. 2A to 2C are views showing light intensity distributions of the two-split sensor elements 8 and 9 when the optical disk 1 is moved in the y direction in FIG. 1. More specifically, FIG. 2A shows a light intensity distribution when the beam spot 4 is located between the groove 3 and the land 2, FIG. 2B shows a light intensity distribution when the beam spot 4 is located on the land 2. FIG. 2C shows a light intensity distribution when the beam spot 4 is located between the land 2 and the groove 3 oppositely to the y direction.
A hatched portion of a light beam 11 on the two-split sensor element 9 in FIG. 2A and a hatched portion of a light beam 11 on the two-split sensor element 8 in FIG. 2C have lower light intensities than those of other portions. If outputs from the two-split sensor elements 8 and 9 are defined as S1 and S2, respectively, a difference signal (S1-S2) is given as S1-S2&gt;0 (A in FIG. 3) in FIG. 2A, S1=S2 (B in FIG. 3) in FIG. 2B, and S1-S2&lt;0 (C in FIG. 3) in FIG. 2C. A difference signal shown in FIG. 3 is normally called a track error signal. The positional relationship between the land 2 and the spot 4 can be detected in accordance with this difference signal. The objective lens 5 is driven in a direction perpendicular to the optical axis so that (S1-S2) becomes zero, thereby accurate tracking may be performed.
The above push-pull method has the following problem. For example, assume that the objective lens 5 is driven in the y direction of FIG. 1 to control the beam spot 4 on a target track. Upon movement of the objective lens 5 in the y direction, the light beams 11 on the two-split sensor elements 8 and 9 are moved in the y direction (although not illustrated). Since a sensor output difference (S1-S2) is also changed upon movement of the light beams, a change in light intensity distribution upon movement of the light beam cannot be distinguished from that upon relative movement between the light beam and the track. This results in an offset of the tracking error signal, and accurate tracking cannot be performed. The above phenomenon also occurs when disturbance such as inclination of the optical disk 1 is effected in a tracking system, thus degrading stability and reliability of the tracking system.